Integrated inductor

ABSTRACT

The invention concerns an integrated inductor ( 20 ), consisting of a flat winding of one or several turns ( 21, 22, 23 ) made of a conductive material above a substrate provided with at least a subjacent conductive level wherein is produced, through a contact pick-up strip ( 12′ ), at least an intersection of the winding, the width of at least one turn and/or one interval between two turns being reduced in line with said contact pick-up strip.

[0001] The present invention relates to the field of integratedcircuits, and more specifically to the manufacturing of an inductanceformed above a semiconductor substrate.

[0002]FIGS. 1A and 1B show, respectively in a top view and in across-section view along line B-B′ of FIG. 1A, a conventional example ofan inductance 1 formed above a semiconductor substrate 2. Inductance 1includes a number of generally concentric turns or spirals (at least onespiral) obtained by the deposition of a conductive element on aninsulating layer 3 (FIG. 1B). Insulating layer 3, for example, siliconoxide, rests on the last metallization level 4 added on substrate 2after forming of integrated components in this substrate. In the exampleof FIG. 1B, two other metallization levels 5, 6 have been illustrated indotted lines between substrate 2 and upper level 4. Each level is ofcourse separated from the underlying level by an insulating layer,respectively 7, 8. The conductive element of inductance 1 isconventionally of constant width and thickness. It is deposited oninsulating layer 3, in the form of a flat winding from a first internalend 10 to a second external end 11.

[0003] To enable connection of inductance 1 to the rest of theintegrated circuit or to a terminal of a package, it is necessary toprovide a contact recovery from internal end 10 of the winding to theoutside of this winding. Conventionally, this contact recovery isobtained by using an underlying metallization level (generally, upperlevel 4). A conductive track 12 (generally rectilinear) is formedtherein between the location under internal end 10 of inductance 1 andthe location under a pad 13 outside of the winding. Pad 13 is formed oninsulating layer 3 in the same conductive material as the winding ofinductance 1. Vias 14 and 15 (for example, made of tungsten)electrically connect end 10 and pad 13 to the respective ends ofunderlying track 12.

[0004] In the forming of an inductance, its quality factor is generallydesired to be optimized. Among the parameters having an influence uponthis quality factor, the parasitic resistance (series resistance) is amajor parameter. To reduce the resistance of the conductive elementagainst the flowing of current, its section is generally desired to bemaximized. Not only the thickness, but also the width of the spirals ofinductance 1 are then increased.

[0005] A disadvantage is that the contact recovery from the internal endof the winding introduces a series resistance that annuls the beneficialeffects of the section increase of this winding. Indeed, the thicknessof the metal levels underlying the winding is imposed by the technologyin which the other components integrated with the inductance aremanufactured.

[0006] For example, the metallization levels are formed in aluminumdeposited over a thickness from 0.8 to 1 μm. The conductive level addedon top of the structure and in which the spirals are formed has, in thecase of aluminum, a thickness on the order of 2.5 μm. However, to avoidadversely affecting the integrated circuit manufacturing, such athickness increase is only conceivable on the last deposited level.

[0007] This problem is posed whatever the number of spirals of theinductance and whatever the conductive materials used. Further, it ismore generally encountered each time a crossing is desired to be made ina winding of an integrated inductance.

[0008] The present invention aims at providing a novel integratedcircuit inductance that overcomes the disadvantages of knowninductances.

[0009] The present invention more specifically aims at solving theproblems associated with the contact recovery from the internal end ofthe inductance.

[0010] More generally, the present invention aims at providing asolution to the problem of crossing, by contact recovery in a lowerlevel, of a flat winding of an inductance.

[0011] To achieve these objects, the present invention provides anintegrated inductance, formed of a flat winding of at least one spiralmade of a semiconductor material above a substrate provided with atleast one underlying conductive level, in which the winding is crossedat least once by a contact recovery track, the spiral width beingreduced above said contact recovery track.

[0012] The present invention also provides an integrated inductance,formed of a winding of several spirals, the width of at least one spiraland/or of at least one interval separating two spirals being reducedabove said contact recovery track.

[0013] According to an embodiment of the present invention, the crossingis used for the contact recovery from an internal end of the winding toan external pad.

[0014] According to an embodiment of the present invention, the patternof the spirals is such that the external spiral is, at the level of thecontact recovery, closer to the center of the winding than the rest ofthis external spiral.

[0015] According to an embodiment of the present invention, theresistance per square of the conductive material constitutive of thewinding is substantially smaller than the resistance per square of theunderlying conductive level in which is formed the contact recovery, thethickness of the conductive material constitutive of the winding being,preferably, substantially greater than the thickness of the underlyingconductive level.

[0016] According to an embodiment of the present invention, theintervals, in the contact recovery alignment, between the two connectedwinding portions, are minimized.

[0017] According to an embodiment of the present invention, the lengthof the narrowed section(s), which depends on the width of the contactrecovery track, is chosen to be as short as possible.

[0018] According to an embodiment of the present invention, theconductive material is aluminum, the underlying conductive level beingalso made of aluminum.

[0019] According to an embodiment of the present invention, saidconductive material is copper having a thickness of several tens ofmicrometers, the underlying conductive level being made of aluminum of athickness on the order of one micrometer.

[0020] According to an embodiment of the present invention, saidconductive level is formed by the upper metallization level used for theinterconnections of other components of the integrated circuit.

[0021] The foregoing objects, features and advantages of the presentinvention will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

[0022]FIGS. 1A and 1B, previously described, respectively show in a topview and in a cross-section view a conventional example of an integratedcircuit inductance;

[0023]FIGS. 2A and 2B show, respectively in a top view and in across-section view, an embodiment of an integrated inductance accordingto the present invention; and

[0024]FIGS. 3A and 3B illustrate, by representations of an integratedinductance, respective in top view and in cross-section view,alternative embodiments of the present invention.

[0025] The same elements have been designated with the same referencesin the different drawings. For clarity, only those elements that arenecessary to the understanding of the present invention have been shownin the drawings and will be described hereafter. In particular, thecomponents with which one or several inductances are integrated on thesemiconductor substrate have not been shown in the drawings and are noobject of the present invention.

[0026] A feature of the present invention is to provide a narrowing ofthe conductive element constitutive of a winding of an integratedinductance, above an underlying conductive track enabling, by a contactrecovery, crossing of the winding. Such a localized narrowing of thewinding spiral(s) enables reducing the length of the underlying contactrecovery section, and thus the series resistance of the inductance. Inan inductance with several spirals, a feature of the present inventionis to provide, above the underlying contact recovery, a narrowing of atleast one spiral of the conductive element and/or of at least oneinsulating interval between spirals.

[0027] The present invention will be described hereafter in relationwith examples of contact recovery of an internal end of the winding.However, all that will be discussed hereafter more generally applies toa crossing at any point of the winding.

[0028]FIGS. 2A and 2B show, respectively in a top view and in across-section view along line B-B′ of FIG. 2A, an embodiment of anintegrated inductance according to the present invention.

[0029] Conventionally, an inductance 20 according to the presentinvention is formed of one or several spirals of a conductive elementdeposited above a semiconductor substrate 2 in which integrated circuitshave been formed. Inductance 20 is deposited flat on an insulating layer3 covering the last metallization level 4 of the integrated circuit. Theexample of FIG. 2B shows the same metal levels 5, 6 and insulatinglevels 7, 8 as in the example previously described in relation with FIG.1B. In the example of FIG. 2A, inductance 20 includes three spirals andone quarter of square shape. However, an inductance according to thepresent invention may have any shape (round, oval, or polygonal) andalso any number of spirals. The use of rectilinear sections howeversimplifies the manufacturing.

[0030] As previously, a contact recovery is provided from internal end10 of the winding to a pad 13 external to this winding. This contactrecovery is performed by means of a conductive track 12′ obtained, forexample, in the last metallization 4 underlying the conductive elementconstitutive of inductance 20. Internal end 10 and pad 13 are connectedto the respective ends of track 12′ by means of vias 14, 15.

[0031] As an alternative, in a circuit with several metallizationlevels, several superposed tracks may be used in the successivemetallization levels to form the contact recovery. These tracks are thenconnected in parallel by means of vias crossing the different insulatinglayers 3, 4, and 8. Indeed, these metallization levels are availablesince the inductance is generally placed on a portion of the substratecontaining no other component. However, this solution leads to usinglevels closer and closer to the substrate, which increases straycapacitances between the winding and the substrate. The choice of thisalternative depends on the desired compromise between the decrease ofthe series resistance and the increase of such capacitances. Accordingto another alternative, the first metallization level used will not bethe level closest to the winding. In this case, the stray capacitancebetween the winding and the contact recovery is decreased.

[0032] According to the embodiment shown in FIGS. 2A and 2B, each spiral21, 22, or 23 that must run above contact recovery track 12′ exhibits,above said track 12′, a narrowed section, respectively 21′, 22′, and23′. As seen from above, narrowing 22′ of intermediary spiral 22 is, forexample, aligned with the rest of the rectilinear section in which it isformed. Narrowings 21′ and 23′ are then not aligned with the rest of thecorresponding sections of spirals 21 and 23 to bring sections 21′ and23′ as close as possible to section 22′. The connection between eachnarrowed section and the rest of the corresponding spiral may have anyshape (for example, oblique, as shown, or with a right angle). Thenarrowings enable reducing the length of track 12′ with respect to asame track which would have to cross sections 21, 22, and 23 in theirnon-narrowed portions. For spirals with rectilinear sections, thenarrowed sections are parallel to one another and, for example,perpendicular to the contact recovery track, the length of which isdesired to be minimized. Accordingly, a contact recovery sectionaccording to the present invention has a resistance smaller than that ofa conventional section in the same technology. By reducing theresistance of the contact recovery section, the general seriesresistance of the inductance is decreased and its quality factor is thusincreased.

[0033] It could have been thought that by narrowing the conductivespirals, the series resistance of the corresponding sections isincreased in such a way that the length decrease of the contact recoverytrack is useless. However, this does not occur. First, the narrowingprovided by the present invention is localized and the shortest possibleto minimize the resistance introduced in each spiral. Further, thisnarrowing does not go along with a thinning down, so that thecross-section of sections 21′, 22′, and 23′ remain relatively large(especially as compared with the underlying level). Moreover, integratedinductances are generally used for high-frequency applications where thecurrent in the inductance is essentially a function of the perimeter ofits cross-section (skin effect). Accordingly, if the spirals aresufficiently thick (thicker than they are wide), the inductance is notdebased by the narrowings provided by the present invention. Taking thepreceding example of an aluminum conductive element of a 2.5-μmthickness placed on a stacking of aluminum metallization levels of 0.8μm, a narrowing down to a 2-μm width (if allowed by the seriesresistance then introduced) can be provided in spirals having for therest a given width ranging, for example, between one and a few tens ofμm.

[0034] In an alternative (not shown) where only the intervals betweenspirals are narrowed above the contact recovery, the series resistanceof the winding is not modified.

[0035] The minimum width of the conductive sections of the inductanceand the intervals between spirals is essentially linked to thetechnological process used according to the thickness of theseconductive sections.

[0036] For example, to improve the conductivity of the inductance,copper, which can then be deposited in a much thicker layer to form theinductance, is also used. Copper thicknesses of several tens of μm (forexample, approximately 30 μm) can then be obtained. With such atechnology, the minimum width and the spacing are approximately half thethickness (that is, for example, approximately 15 μm). The width of thenon-narrowed sections is for 25′ example on the order of 30 to 40 μm.The underlying metallization levels remain, for example, made ofaluminum.

[0037] Of course, the nature of the material and/or its thickness may bemodified. What matters is that the resistance per square of theconductive element of the winding is smaller than the resistance persquare of the metallization level containing the contact recovery.

[0038]FIGS. 3A and 3B illustrates alternative embodiments of aninductance according to the present invention. FIG. 3A is a top view andFIG. 3B is a cross-section view along line B-B′ of FIG. 3A.

[0039] The embodiment illustrated in FIGS. 3A and 3B is a hexagonalinductance 30 having four spirals and one third, formed of rectilinearsections. In addition to the shape variation, this embodiment includesanother variation in the arrangement of narrowings 31′, 32′, 33′, and34′ of spirals 31, 32, 33, and 34 above track 12′ of transfer ofinternal end 10 of inductance 30 to external pad 13. In the embodimentof FIGS. 3A and 3B, these narrowings are brought as close as possible tointernal end 10 of inductance 30, while in the embodiment of FIGS. 2Aand 2B, these narrowings enable bringing spirals 21 and 23 closersymmetrically with respect to second spiral 22. Other alternatives arepossible. For example, its may be provided to bring the narrowedsections closer to external spiral 34 rather than to internal spiral 31.In this case, internal end 10 of the inductance is also brought closerto external spiral 34.

[0040] According to another embodiment not shown, the inductance is saidto be “symmetrical” and includes a crossing approximately equidistantfrom the ends of the winding which are both outside of it. In the caseof a winding with several spirals, there are then several crossings,each contact recovery running under a single spiral.

[0041] An advantage of the present invention is that it reduces theseries resistance of the inductance with respect to a conventionalinductance. On the one hand, by reducing the length of the contactrecovery track, the capacitance between said track and the substrate isreduced. On the other hand, by narrowing the spirals, the capacitancebetween the winding and the contact recovery is reduced.

[0042] Another advantage of the present invention is that it decreasesthe integrated circuit surface area in which the inductance isinscribed. Indeed, by bringing the spirals closer towards the inside atthe level of their narrowing, external connection pad 13 is broughtcloser to the center of the winding. This advantage especially appearsfrom FIG. 3A.

[0043] Of course, the present invention is likely to have variousalterations, modifications, and improvements which will readily occur tothose skilled in the art. In particular, the respective sizing of theinductance sections, of their narrowing and of the underlying contactrecovery track depend on the application and are to be adapted case bycase by those skilled in the art. Further, the contact recovery levelmay be formed by any metallization or polysilicon level, or even by asubstrate region. Finally, the winding itself may include severalconductive levels in parallel (connected by vias) provided that thewinding has, at least in one of these levels, a resistance per unitlength smaller than that of the contact recovery.

1. An integrated inductance (20; 30), formed of a flat winding of atleast one turn (21, 22, 23; 31, 32, 33, 34) made of a semiconductormaterial above a substrate (2) provided with at least one underlyingconductive level (4), in which the winding is crossed at least once by acontact recovery track (12′), characterized in that the width of thewinding portion above said contact recovery track is reduced.
 2. Anintegrated inductance (20; 30), formed of a winding of several turns(21, 22, 23; 31, 32, 33, 34) of a semiconductor material above asubstrate (2) provided with at least one underlying conductive level(4), in which the winding is crossed at least once by a contact recoverytrack (12′), characterized in that the width of at least one windingportion and/or of at least one interval separating two turns above saidcontact recovery track is reduced.
 3. The inductance of claim 1 or 2,characterized in that the contact recovery track extends from aninternal end (10) of the winding to an external pad (13).
 4. Theinductance of claims 2 and 3, characterized in that the pattern of thewinding (21, 22, 23; 31, 32, 33, 34) is such that the external turn is,at the level of the contact recovery (12′), closer to the center of thewinding than the rest of this external turn.
 5. The inductance of any ofclaims 1 to 4, characterized in that the resistance per square of theconductive material constitutive of the winding is substantially smallerthan the resistance per square of the underlying conductive level (4) inwhich is formed the contact recovery (12′), the thickness of theconductive material constitutive of the winding being, preferably,substantially greater than the thickness of the underlying conductivelevel.
 6. The inductance of any of claims 1 to 5, characterized in thatthe intervals, in the contact recovery alignment, between the twoconnected winding portions, are minimized.
 7. The inductance of any ofclaims 1 to 6, characterized in that the length of the narrowedportion(s) (21′, 22′, 23′; 31′, 32′, 33′, 34′), which depends on thewidth of the contact recovery track (14′), is chosen to be as short aspossible.
 8. The inductance of any of claims 1 to 7, characterized inthat the conductive material is aluminum, the underlying conductivelevel (4) being also made of aluminum.
 9. The inductance of any ofclaims 1 to 8, characterized in that said conductive material is copperhaving a thickness of several tens of micrometers, the underlyingconductive level (4) being made of aluminum of a thickness on the orderof one micrometer.
 10. The inductance of any of claims 1 to 9,characterized in that said conductive level (4) is formed by the uppermetallization level used for the interconnections of other components ofthe integrated circuit.